The turf is what composes the surface of the playing field. At its simplest, the turf can be just dirt or crushed pebbles (mainly for Little League of softball fields). However, the areas that are covered by a form of grass - either natural grass or artificial turf are considered the turf.
The nature and quality of the turf can affect the playing of the game, by allowing ground balls to roll faster or slower, or by allowing high or low bounces on batted balls. The turf can also affect base stealing. It also has an impact on the comfort of the players, as, for example, artificial turf will tend to trap in heat more than natural grass, but will also be a lot easier to maintain in rainy and wet conditions. The types injuries suffered on different types of turf will also vary.
Ballparks have been known to switch the type of turf they use from one season to another, in order to improve groundskeeping.